1. Field of the Invention
The present invention relates to a dielectric-coated electrode such that the surface of a conductive base material is coated with a dielectric, to a plasma discharge treatment apparatus comprising the dielectric-coated electrode, and a to method for forming a thin film by using the dielectric-coated electrode.
2. Description of Related Art
Recently, various high-functional films such as conductive films, antireflection films, antistatic films and the like have been used for liquid crystal display elements, semiconductor elements, optical elements and the like. As for a method for forming these high-functional films, a method for forming a thin film on a surface of a substrate or the like disposed between electrodes by discharging under atmospheric pressure or under a pressure in the vicinity of atmospheric pressure and performing plasma excitation of a reactive gas (atmospheric pressure plasma method) is disclosed in the Japanese Patent Laid-Open Publications Nos. 11-133205, 2000-185362, 11-61406, 2000-147209, 2000-121804 and the like.
The atmospheric pressure plasma method disclosed in these Publications is for generating discharge plasma between facing electrodes by applying a voltage having a frequency of 0.5 to 100 kHz so that the electric field strength will become from 1 to 100 V/cm.
Incidentally, as for the electrodes used in such an atmospheric pressure plasma method, the surface of the electrodes is required to be coated with a dielectric in order to obtain stable glow discharge under atmospheric pressure. There is no limitation in this dielectric unless it has a desired dielectric constant. However, a rubber, ceramic, glass and the like are generally known.
A technique for using a dielectric-coated electrode such that an alumina, which is a ceramic, is sprayed on a metal base material in an atmospheric pressure plasma treatment apparatus is suggested in the Japanese Patent Laid-Open Publication No. 11-191500. Such a dielectric-coated electrode has high durability rather than an electrode simply having a dielectric of rubber or glass. Such an electrode has no problem in so-called “surface treatment” such as changing a surface property such as wettability, adhesiveness or the like of the surface of a substrate, removing dirt on the surface of a substrate, or the like.
However, when the present inventor has studied the case of forming a high performance thin film in a large area, it is realized that it is required to apply a high-frequency voltage having higher frequency and larger electric power compared with the above-described surface treatment and that the durability of the above-described dielectric-coated electrode to which only an alumina is sprayed is insufficient. Concretely, it is recognized that a high-frequency voltage having a frequency over 100 kHz and electric power of not less than 100 W/cm2 is required to be applied between facing dielectric-coated electrodes in an atmospheric pressure plasma treatment apparatus. Therefore, an electrode having high durability, which is durable in applying such a voltage of high power, has been studied earnestly. As a result, a dielectric-coated electrode capable of maintaining a discharged state which is stable for a long term even though a voltage of high power is applied has been found.